1. Field of the Invention
The invention relates in general to the communications field and, more particularly, to a system and method capable of locking onto ultra-wideband signals in a multipath environment.
2. Description of Related Art
Recent advances in communications technology have enabled an emerging and revolutionary ultra-wideband technology (UWB) called impulse radio communications systems (hereinafter called impulse radio). Basic impulse radio transmitters emit short pulses approaching a Gaussian monocycle with tightly controlled pulse-to-pulse intervals. Impulse radio systems typically use pulse position modulation, which is a form of time modulation where the value of each instantaneous sample of a modulating signal is caused to modulate the position of a pulse in time.
For impulse radio communications, the pulse-to-pulse interval is varied on a pulse-by-pulse basis by two components: an information component and a pseudo-random code component. Unlike direct sequence spread spectrum systems, the pseudo-random code for impulse radio communications is not necessary for energy spreading because the monocycle pulses themselves have an inherently wide bandwidth. Instead, the pseudo-random code of an impulse radio system is used for channelization, energy smoothing in the frequency domain, and interference suppression.
Generally speaking, an impulse radio receiver is a direct conversion receiver with a cross-correlator front end. The front end coherently converts an electromagnetic pulse train of monocycle pulses to a baseband signal in a single stage. The data rate of the impulse radio transmission is typically a fraction of the periodic timing signal used as a time base. Because each data bit modulates the time position of many pulses of the periodic timing signal, this yields a modulated, coded timing signal that comprises a train of identically shaped pulses for each single data bit. As an option, the impulse radio receiver can integrate multiple pulses to recover the transmitted information.
In a multi-user environment, impulse radio depends, in part, on processing gain to achieve rejection of unwanted signals. Because of the extremely high processing gain achievable with impulse radio, much higher dynamic ranges are possible than are commonly achieved with other spread spectrum methods.
When receiving an ultra-wideband signal as part of a one-way communication system (e.g., a pager) or a two-way communication system (e.g., a wireless telephone), a problem exists as to how to detect the transmitted impulse radio signal, and more particularly, how to acquire and lock onto the impulse radio signal being transmitted. This problem exists independent of how the received ultra-wideband signal is used, such as in a one-way or two-way communication system.
Previous approaches to solving this problem are discussed in the following commonly owned patents, which are incorporated by reference: U.S. Pat. No. 5,832,035, issued Nov. 3, 1998 to Fullerton, and U.S. Pat. No. 5,677,927, issued Oct. 14, 1997. The present invention presents another approach to solving this problem.